Many digital radio communication systems, such as cellular, cordless and data transmission systems, use FSK, GFSK (Gaussian Frequency Shift Key), or GMSK (Gaussian Mean Shift Key) modulation techniques. These types of modulation techniques are in fact simply frequency modulation (FM) with the radio frequency (RF) signal envelope constant.
Since there is no amplitude modulation (AM) involved in these types of modulation techniques, the voltage controlled oscillator (VCO) frequency of the transmitters can be directly modulated by the baseband signal, as is typical in regular analog FM transmitters, such as in analog cellular systems. Significant cost reductions can be obtained by directly modulating the VCO frequency. Such arrangements are particularly desirable in digital applications where low cost is a strategic factor. For example, the overall cost of a digital solution such as DECT must be very low to be competitive with well known analog systems such as CT0.
In order to avoid any inter symbol interferences which could corrupt the eyes pattern and degrade the bit error rate of the transmission in the digital system, the amplitude transfer function and the group delay on the modulation path has to be kept constant across the entire spectrum of the baseband signal. This requires the transfer function of the phase lock loop (PLL) to be high enough to pass the entire modulation spectrum. Furthermore, in order to meet the applicable radio specifications, the spectral purity of the RF signal source to be transmitted should be maintained as near to the carrier signal as possible for phase noise and modulation accuracy, and should be maintained as far from the carrier signal as possible to reduce harmonics, the noise floor, and discrete spurious signals. As a consequence, the transfer function of the PLL must be low enough to filter the noise.
A technique called Dual Port modulation is known to provide the capability to generate a low port modulation signal as well as a high port modulation. The low port modulation signal is used to drive a noise shaping circuit that controls the divider of the PLL, while at the same time the high port frequency modulation signal is used as a input to a high port path which utilizes a digital to analog converter (DAC) to directly drive the input voltage of a voltage controlled oscillator (VCO). The effect of the dual port modulation is to provide a low pass transfer function for the generated noise of the reference clock and the noise shaping circuitry, and to provide an all pass function to the input frequency deviation.
Existing dual port modulation circuits have several problems. First, directly coupling the DAC output to the VCO input can produce noise at the output of the VCO. Second, a smoothing filter at the DAC ouput is needed in order to filter the analog output produced by the DAC. And third, several look-up tables are required to store the frequency deviation pulse shaping values for a given frequency modulation. Some frequency modulations require different crystal clocks to drive the PLL, or for a given crystal clock require different reference clock for the PLL, in order to provide different band ranges (e.g. private mobile radio systems). If the reference clock has to be modified, the number of look-up tables must be increased for various reference clocks, which then results in an increased system cost.